LONDON (Reuters) - Longer Arctic summers and thinning sea ice are threatening the habitats of polar bears and the livelihood of native people, scientists said Wednesday.

Arctic ice has lost as much as 40 percent of its thickness in the past 50 years.

But researchers, who measured it from space for the first time, said it varies more widely than previously thought and is mainly because of summer melting.

"We found a direct link between lengthening summers and thinning ice," said Seymour Laxon of University College London.

The thinning sea ice will not cause sea levels to rise but it could have a knock-on effect for the climate, Arctic ecosystems and wildlife.

"There is a lot of concern about the effect on the people in the high Arctic. They rely on the ice to do their hunting and fishing and each year the ice season is getting shorter and shorter," Laxon said.

"They are very concerned about how that is going to change their way of life," he added in an interview.